1. Field of the Invention
This invention relates to a developing assembly used in image-forming apparatus such as copying machine and laser beam printers making use of an electrophotographic system or electrostatic recording system in which a developer is made to adhere to an electrostatic latent image formed on an image-bearing member, to render it visible. More particularly, it relates to an improvement of a developer-carrying member used to transport the developer.
2. Related Background Art
In forming color images, two-component developers are commonly used which have non-magnetic toner particles and a magnetic carrier. As a developing assembly used in an image-forming apparatus making use of such a known two-component developer, known is as shown in FIG. 2, a developing assembly 9 having a developer container 9a which holds therein the developer, a developer-carrying member 3 which is rotatably provided at an opening of the developer container 9a and is disposed facing an image-bearing member 4 to hold thereon and transport the developer to a developing zone which is the part where it faces the image-bearing member 4, a developer layer thickness control member 2 which is disposed in a non-contact state with respect to the developer-carrying member 3 to control (regulate) the layer thickness of the developer held on the developer-carrying member 3, a magnetic-field formation means 13 which is stationarily disposed inside the developer-carrying member 3 and by which a magnetic field for drawing up the developer held in the developer container 9a and controlling the layer thickness of the developer is formed at the part where it faces the developer layer thickness control member 2, and agitation means 11 and 12 which agitate the developer held in the developer container 9a. 
The two-component developer held in the developer container 9a is so agitated as to circulate reciprocatingly in the developer container 9a by the agitation means 11 and 12. Also, the two-component developer held in the developer container 9a is drawn up onto the developer-carrying member 3 by the magnetic-field formation means 13 and is transported to an S2 pole, to an N3 pole and to an S1 pole as the developer-carrying member 3 is rotated. In the course of this transport, the developer is controlled against the developer-carrying member 3 by the developer layer thickness control member 2 in the vicinity of the S2 pole, so that a thin layer of the developer is formed on the developer-carrying member 3. This developer rises in ears by the action of the S1 pole and develops the electrostatic latent image on the image-bearing member 4. Thereafter, its portion having not participated in the development falls from the developer-carrying member 3 to the developer container 9a by the action of a repulsion magnetic field formed by the N1 pole and N2 pole, and is collected there.
Developers for forming color images (non-magnetic toner particles) are required to have superior transparency of fixed images and various superior properties in respect of low-temperature fixing performance, high-temperature anti-offset properties and so forth. As color toner particles having intended for such attempts, toner particles having a specific storage elasticity are known as disclosed in, e.g., Japanese Patent Application Laid-open No. 11-84716, and toner particles containing a wax are known as disclosed in, e.g., Japanese Patent Application Laid-open No. 5-61238.
Meanwhile, in respect of improvement in the transparency of fixed images and the high-temperature anti-offset properties, techniques which intend to improve them by how to fix images are known in the art. For example, a method is known in which an oil such as silicone oil or fluorine oil is applied on a heat fixing roller. However, fixed images thus obtained have any excess oil adhering to their surfaces. Also, the oil may adhere to the image-bearing member to contaminate it, or the oil may swell the fixing roller to shorten the lifetime of the fixing roller. It is also necessary to feed the oil uniformly and at a constant rate to the surface of the fixing roller so that any oil streaks may not appear on the fixed images. This tends to make the fixing assembly have a large size. Thus, various problems are pointed out in the above method. From such viewpoints, the fixing making use of oils is not preferable in some cases. Accordingly, in order to perform oil-less fixing, toner particles containing a wax may be used. This is advantageous to make up toner particles showing good properties.
As the developer-carrying member in the developing assembly making use of a two-component developer, known is a developer-carrying member the surface of which has been subjected to surface-roughing in order to achieve formation of a uniform developer layer and improvement in transport performance for the developer. As a method of surface-roughing the developer-carrying member surface, amorphous blasting (sand blasting) making use of amorphous particles, having no uniformed shape, is conventionally chiefly carried out in the case of two-component development systems.
The amorphous blasting is a method in which the surface of metal is made to have fine unevenness by spraying at a high speed, sand, alumina particles, silicon oxide particles or the like which have sharp angles and are amorphous. The developer-carrying member surface having been subjected to amorphous blasting is, in its magnified observation, in a rough state that fine grooves are innumerably present. The surface of the developer-carrying member having been subjected to amorphous blasting in this way comes improved in developer transport performance as having been surface-roughed.
However, in color image-forming apparatus and where a two-component developer having non-magnetic toner particles containing a wax is used in a developing assembly having a developer-carrying member having subjected to such amorphous blasting, the following problems are pointed out.
In the conventional developing assembly shown in FIG. 2, the developer having been controlled by the developer layer thickness control member 2 to come left behind is held by the magnetic force of the S2 pole and N2 pole in the vicinity of the upstream side of the developer-carrying member in its rotational direction and gathers there in a large quantity, where the developer is further successively transported as the developer-carrying member 3 is rotated. Hence, a great pressure tends to be applied to the developer on its side upstream to the developer layer thickness control member 2. This tends to cause deterioration of the developer and adhesion of toner particles to magnetic-carrier particle surfaces (what is called xe2x80x9ctoner-spent phenomenonxe2x80x9d) as a result of service for a long time. Occurrence of such a phenomenon tends to change the charge quantity (what is called xe2x80x9ctriboelectricityxe2x80x9d) of non-magnetic toner particles with service time to cause a change in image density, and images with a very poor impression may come to be formed as the developer is used, compared with images at the initial stage.
The developer containing a wax also commonly tends to cause adhesion of non-magnetic toner particles to the surfaces of the developer-carrying member and magnetic carrier particles, compared with developers not containing any wax, and tends to make the developer have a poor fluidity or cause melt adhesion of toner particles to the developer-carrying member surface (hereinafter also xe2x80x9csleeve contaminationxe2x80x9d). This tends more remarkably especially in the cases of, e.g., pulverization toner particles on the toner particle surfaces of which the wax tends to be present and toner particles containing the wax in a large quantity.
The developer-carrying member having been subjected to amorphous blasting also has a surface formed in such a surface state that fine grooves are present in a large number. Hence, when such a developer-carrying member is used, the non-magnetic toner particles or any components in the toner particles tend to be caught in dales of the fine unevenness of the developer-carrying member surface to come adhered thereto. The toner particles having adhered to the developer-carrying member surface have a possibility of coming to melt-adhere thereto because of pressure and so forth applied when the layer thickness of the developer is controlled.
In particular, with an increase in the demand for color copying machines or the like in recent years, the copying machines are demanded to achieve a higher image quality and also demanded to have a lower power consumption, in accordance with which the non-magnetic toner particles are made to have a smaller particle diameter and a lower softening point. With such trends, the non-magnetic toner particles or any components in the toner particles more tend to melt-adhere to the uneven area of the developer-carrying member surface as a result of long-term use to come to cause sleeve contamination.
Occurrence of the melt adhesion of toner to the developer-carrying member surface may first make small the quantity in which the developer is transported to the developing zone, to tend to cause a decrease in image density. Also, in conventional cases, in order to perform good development, an alternating electric field (development bias) is often applied to the developer-carrying member at the time of development. In such a case, on occurrence of the melt adhesion of toner to the developer-carrying member surface, a high-resistance layer due to molten deposits may inevitably be formed on the developer-carrying member surface, so that no desired electric field may be formed at the developing zone between the developer-carrying member and the image-bearing member at the time of development. As the result, any sufficient development effect attributable to the development bias cannot be obtained to cause a decrease in image density or cause faulty images such as blank areas.
An object of the present invention is to provide a developing assembly and an image-forming apparatus which have made it possible, in image formation making use of a two-component developer having non-magnetic toner particles and a magnetic carrier, to weaken the pressure applied to the two-component developer to prevent the non-magnetic toner particles from burying in or melt-adhering to the developer-carrying member surface and prevent the sleeve contamination so that good transport performance for the two-component developer by the developer-carrying member can be maintained and thereby images with good quality can stably be formed over a long period of time.
More specifically, the present invention provides a developing assembly comprising:
i) a developer container which holds therein a developer;
ii) a developer-carrying member which is rotatably provided at an opening of the developer container to hold thereon the developer and transport the developer to a developing zone;
iii) a developer layer thickness control member which is disposed in non-contact with the developer-carrying member to control the layer thickness of the developer held on the developer-carrying member;
iv) a magnetic-field formation means which is enclosed inside the developer-carrying member and disposed stationarily to the developer container and which has magnetic poles for simultaneously a) drawing up the developer from the developer container to the developer-carrying member and b) forming a magnetic field for controlling the layer thickness of the developer on the developer-carrying member at its part where it faces the developer layer thickness control member; and
v) an agitation means which agitates the developer held in the developer container;
wherein the developer is a two-component developer which contains wax-dispersed non-magnetic toner particles and a matnetic carrier, the wax-dispersed, non-magnetic toner particles contain at least a binder resin, a colorant, an organometallic compound and a wax, and have an average particle diameter in a range fo 2 xcexcm to 10 xcexcm; and
the developer-carrying member having been subjected to surface blasting with spherical particles to have a surface having a ten-point average roughness (Rz) of from D/6 to D/2 where D is the average particle diameter of the magnetic carrier.
The present invention also provides an image-forming apparatus making use of the above developing assembly.
According to the above construction, a developing assembly and an image-forming apparatus can be provided which have made it possible, in image formation making use of the two-component developer having non-magnetic toner particles and a magnetic carrier, to weaken the pressure applied to the two-component developer to prevent the wax-dispersed non-magnetic toner particles from burying in or melt-adhering to the developer-carrying member surface and prevent the sleeve contamination so that good transport performance for the two-component developer by the developer-carrying member can be maintained and thereby images with good quality can stably be formed over a long period of time. Thus, the object of the present invention is achieved.